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Palm
Disease Update
Fusarium Decline of Queen Palms and Mexican Fan Palms in Florida
Palm Diseases Caused by Phytoplasmas
Palm Diseases Caused by Phytoplasmas: FAQ Sheet
July
2007 Update
Monica L. Elliott, Ph.D.
University of Florida – IFAS, Fort Lauderdale Research and Education
Center
For those of you familiar
with previous updates, you might recall the title was “New Disease of
Queen Palms (Syagrus romanzoffiana) in the South Florida Landscape.” The
change in the title
reflects additional information obtained since the last update prepared
in May 2006. This
document is a summary of information regarding this new disease. If you
only want to know the
“bottom line,” please fast forward to the summary.
Why has the title changed? First, I am feeling more confident with each
pathogenicity
experiment that the pathogen is Fusarium oxysporum. I am tentatively
naming the new disease
“Fusarium decline.” Second, a second host has been identified,
Washingtonia robusta (Mexican
fan palm). Third, while the disease is still primarily observed in
landscapes, container and field
nurseries are also now documented as disease sites. Fourth, while the
disease has not been
documented in north Florida, it seems likely that the disease will occur
in other locations besides
the southern half of the state.
History
The sporadic death of queen palms in one Palm Beach County was first
brought to our attention
in 2003. Conversations with landscape maintenance staff throughout the
southern half of the
state, where most queen palms are grown, indicated the problem was
observed in other counties
at approximately the same time. This meant the 2004 and 2005 hurricane
seasons could not be
blamed for the spread of the problem. While initially less than 1% of
the queen palms died in a
single landscape, there are now sites where upwards of 5-10% of the
queen palms have died.
Until early 2007, no other palm species had been affected, even in
landscapes dominated by a
mixture of palm species. Most of the queen palms observed had been in
the landscape for five or
more years – i.e., it was affecting established palms and not newly
transplanted plants. Only one
nursery had been affected until early 2007, and it was a container
nursery and not a field nursery.
Host Expansion
In late April/early May 2007, I obtained samples of diseased leaves from
Washingtonia robusta
from three locations distant to each other – Lee County, Orange County
and Pinellas County.
Two samples were from palms established in the landscape and one sample
was from a field
nursery. One of the landscape palms was on a property site where the
disease had been observed
previously on queen palms. The field nursery had never observed the
disease prior to this time,
and the disease occurred simultaneously in a queen palm block and a
Mexican fan palm block
(situated side by side). These palms had been in the field for
approximately two years.
Probable Pathogen (or Patience is a virtue!)
I have consistently isolated Fusarium oxysporum from diseased queen
palms (mature with
trunks) in the landscape since early 2004. However, it was not uncommon
for other “potential”
pathogens to be isolated at the same time and from the same small piece
of tissue, including
other Fusarium species. On the other hand, Fusarium oxysporum has been
the only “potential”
pathogen isolated from the petiole tissue of diseased queen palms
growing in a container nursery
and, most recently, from diseased queen and Mexican fan palms growing in
a field nursery
(juveniles without trunks).
To confirm which fungus isolated from a diseased plant is the pathogen,
plant pathologists must
complete a series of steps called Koch’s postulates. The first step is
consistent isolation of a
pathogen from symptomatic host tissue. In the second step, the pathogen
is grown in pure
culture and its characteristics documented. The third step is
inoculation of a healthy plant with
the pure culture of the pathogen, using the same plant species from
which the pathogen was
originally isolated. This inoculated plant must then develop symptoms
similar to those observed
initially. The fourth step is recovery of the same fungus used for
inoculation purposes.
Sometimes completing Koch’s postulates is a simple task; sometimes it
isn’t! It has been
extremely difficult to complete Koch’s postulates with the Fusarium
oxysporum and queen
palms. We have set-up a series of experiments using queen palms
(seedlings and juveniles) and
thus far, IF the palms die, they are dying from Fusarium oxysporum and
not from other
“potential” pathogens. On the other hand, when Washingtonia robusta
seedlings were inoculated
the first time (using the exact same isolates as used for the queen
palms seedlings), all inoculated
plants died within a month.
Preliminary DNA analysis indicates that the Fusarium oxysporum isolates
obtained throughout
the state and from both palm hosts are the same. Furthermore, this
Fusarium oxysporum appears
to be new. It does not appear to match any other known pathogenic
Fusarium oxysporum strains,
including the strains that cause Fusarium wilt of Canary Island date
palm, edible date palm and
oil palm. Further DNA analysis is needed to confirm these preliminary
results.
Pathogen x Host(s)
Fusarium oxysporum is not a single fungus, but is composed of subgroups
called formae
speciales. In general, these subgroups are determined based on the host
species affected by the
fungus because these subgroups cannot be separated based on the typical
characteristics used to
separate fungi. Examples: The primary host of Fusarium oxysporum f. sp.
canariensis is
Phoenix canariensis (Canary Island date palm). The only host of Fusarium
oxysporum f. sp.
albedias is Phoenix dactylifera (date palm), and this fungus is only
found in Morocco and
Algeria. The primary host of Fusarium oxysporum f. sp. asparagi is
Asparagus officinalis
(asparagus). Thus, it is quite surprising, but not impossible, to have
isolated the same Fusarium
oxysporum from two different palm species. Whether other palm species
will be affected is
simply not known.
However, this is an alarming development relevant to management
strategies and potential
spread of the pathogen. If queen palms had been the only host, then
avoiding the planting of
queen palms into an affected area would have been a useful long-term
strategy. While we have
observed the continuing northward movement of queen palms (they really
are not very cold
hardy) with some dismay, Washingtonia robusta are relatively cold hardy
and are found
throughout the state. This means the disease can spread throughout the
state. In other words, it
is no longer just a south Florida problem.
Symptoms
The problem has always appeared to be a disease. No insects are observed
in association with
symptomatic palms, nor are nutritional deficiencies implicated in the
problem.
• Queen palm
Symptoms on queen palms (a featherleaf palm) are as follows. The lowest
(oldest) 2-3 leaves
turn brown (leaflets and petiole) but do not break or hang down (Figure
1). The next 2-3
youngest leaves in the canopy will turn varying shades of yellow. This
pattern continues up
through the canopy (Figure 2) until the entire canopy is brown. It looks
as if the canopy has been
freeze-dried in place (Figures 3 and 4). The time between initial
symptoms and palm death is
only a few months. The symptoms are different from early senescence of
leaves with potassium
deficiency, a common nutrient deficiency of queen palms. With severe
potassium deficiency, the
leaflets will be brown but the petiole will be green. With the new
disease, both the leaflets and
the petiole are brown.
Closer examination of the yellowing leaves and the next green leaf in
the canopy should reveal
what is believed to be the initial target of the pathogen – the leaf
petiole at the point where it is
bending out of the canopy. There are areas of discoloration
(brownish-red color) at this point
that seem to spread in both directions on the petiole (a stripe), toward
the trunk and toward the
leaf tip (Figure 5). The petiole is not soft and rotted, but simply
discolored. Cross-sections
through the petiole reveal internal discoloration (Figure 6).
Cross-sections must be made with a
sharp saw and not with a crushing tool such as pruning shears or
loppers, as the crushing motion
will discolor tissue also. Leaflet tips, even on lower green leaves, may
exhibit drought-like
symptoms. I have also noted that many leaves initially exhibit one-sided
death symptoms, where
the leaflets on one side of the leaf are brown (necrotic) but the
leaflets on the other side of the
leaf are green, with the brownish-red stripe on the petiole
corresponding to the side with the dead
leaflets (Figure 7).
• Mexican fan palm
Symptoms on Mexican fan palms will also have more brown (necrotic) lower
leaves than normal
and subsequently younger leaves with leaf blades that are partly green
and partly yellow or
brown (Figure 8). The petioles of these leaves will have a brown to
reddish-brown stripe
running from the blade back to the trunk (Figure 9). Again, it appears
to be a relatively short
time (a few months) between initial symptom development and palm death.
Cross-sections
through the striped petiole reveals an associated internal
discoloration. The individual leaf
symptoms (brown stripe with uneven leaf blade coloration) will look the
same as another disease
that can be observed on Mexican fan palms, petiole blight. Petiole
blight is not normally a fatal
disease, whereas Fusarium decline is fatal.
For both palms, the apical meristem (bud) of the palm is not killed
until sometime after the
canopy turns completely brown. When cross-sections are made through the
crown of a dying
queen palm, the bud is still clean and white, but older leaf and
inflorescence (flower) bases are
discolored and usually rotted. The symptoms and their development
suggest the pathogen is
producing a toxin.
Pathogen Movement
Based on the locations of the disease (both on queen palms and now
Mexican fan palms)
throughout the southern half of the state, it would appear that the
pathogen is most likely spread
to new sites by wind. Once the disease is established in a landscape or
neighborhood or nursery,
it is possible the pathogen could be spread by pruning tools, in
addition to being spread by wind.
Therefore, as with Fusarium wilt of Canary Island date palms, it may be
useful to sterilize
pruning tools prior to pruning queen palms and Mexican fan palms, even
those that appear
healthy. If the client insists on having leaves of a diseased palm
trimmed (rather than removing
the entire palm immediately), sterilize the pruning tool prior to
pruning the next palm. See the
table below for materials to use for sterilizing pruning tools.
Management Strategies
Palms with Fusarium decline should be removed immediately and the canopy
destroyed. As
long as the disease has not moved down into the trunk tissue, the trunk
can be recycled as mulch.
Removing the diseased palm removes a potential inoculum source of the
disease. However,
caution should be used regarding Mexican fan palms. Without a laboratory
test, it is not possible
to know if a Mexican fan palm has Fusarium decline or petiole blight. If
it has petiole blight, the
palm does not need to be removed, as it is not likely to die. Until more
is known about the
disease, it would not be recommended to plant a queen palm or Mexican
fan palm back into the
site where the diseased palm was located.
No fungicides are recommended. Curative applications would not be
feasible since the palms
decline so quickly after the disease symptoms are observed. Also,
fungicides have not been
shown to be effective in controlling similar diseases such as Fusarium
wilt of Canary Island date
palms.
Photographs illustrating symptoms can be found at the following website:
http://flrec.ifas.ufl.edu/palm_prod/fusarium.shtml
Summary
• The quick-killing disease of queen palms (Syagrus romanozoffiana) has
spread to another
host, the Mexican fan palm (Washingtonia robusta).
• The disease has been observed in landscapes, a container nursery and a
field nursery.
• The disease has been documented thus far only in the southern half of
the state (Orlando
to Tampa to Cocoa Beach and all areas south – inland and coastal).
However, it is quite
likely the disease will spread further north over time.
• Infected palms die quickly, often within a few months of the initial
symptoms.
• There is no cure once a palm is infected, and no preventive fungicide
treatments are
recommended.
• The disease is probably spread by wind into new sites. Once
established, it could also be
spread by pruning tools. Pruning tools should be sterilized after each
palm is trimmed.
• Palms, especially queen palms and Mexican fan palms, should not be
replanted into a site
where a palm with this disease was removed.
• The disease is tentatively called Fusarium decline of palms. The
fungus Fusarium
oxysporum appears to be the pathogen.
Table 1. Suggested materials and soaking times for disinfecting pruning
tools. Information
developed for managing Fusarium wilt of Canary Island date palm.
Materialz % Solution Soaking time
Household bleach (ex: Chlorox) 25% (1 part bleach + 3 parts water) 5
minutes minimum
Pine oil cleaner (ex: Pine Sol) 25% (1 part cleaner + 3 parts water) 5
minutes minimum
Rubbing alcohol (70% isopropyl) 50% (1 part alcohol + 1 part water) 5
minutes minimum
Denatured ethanol (95%) 50% (1 part alcohol + 1 part water) 5 minutes
minimum
zThe above materials were shown to be effective in eliminating the
fungus from the wood dust or
palm sap trapped on pruning tools (Simone, 1998). It is suggested that
the solution be replaced
after ten trees or every two hours. Rinse the tool with fresh water
after disinfecting. Other
potential disinfectants would be trisodium phosphate or quaternary
ammonium salts. The latter
is recommended as a 5% solution, soaking for 5 minutes (Smith et al.,
2003).
Palm Diseases Caused by Phytoplasmas
Palm Diseases Caused by Phytoplasmas: FAQ Sheet
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